This invention relates to an improvement in the internal structure of a cathode ray tube and more particularly to improved shielding means for minimizing the effects of neck charge on the beam trajectory within the electron gun assembly of the tube.
The advance of cathode ray tube technology has introduced a trend toward miniaturization and compaction of electron gun structures, such structures in turn being encompassed within envelope neck portions of smaller diametrical dimensions. Consequently, the spacings between the electrode components of the electron gun assembly and the adjacent sidewall of the enclosing neck portion of the envelope have of necessity become increasingly smaller. This constructional condition is a consideration of prime importance in multi-beam color cathode ray tubes, especially in the types employing in-line electron gun construction wherein the side-oriented electrodes in the gun assembly are particularly close to the wall of the neck.
The presence of an electrical charge on the interior surface of the envelope neck portion, particularly in the region encompassing the electron gun assembly, is a known and elusive phenomenon associated with both the processing and operation of the tube. While all aspects of the neck charge condition are not fully understood, it appears to be influenced by the presence of minute sublimations deposited on the interior surface of the glass during stages of tube processing and subsequent operation. While glass per se will retain an electrical charge, the additional presence of a minute film of sublimated contaminants thereover tends it aggravate the neck charging condition.
In cathode ray tube construction it is usual practice for the funnel-disposed conductive coating to extend into the forward region of the contiguously integrated neck portion. Normally, this coating is of the same high positive electrical potential as that of the final electrode of the electron gun assembly. The adjacency of this high potential conductive coating which ends in the forward region of the neck portion, in conjunction with the possible presence of contamination on the glass surrounding the forward portion of the electron gun assembly, creates a condition fostering the build-up of a positive electrical charge on the interior surface of the glass in that region of the neck.
In the forward portion of the electron gun assembly, the inter-electrode spacing between the focusing and high voltage accelerating electrodes is a vulnerable region through which the neck charge may influence the trajectory of the electron beam traversing the respective electrodes. For example, in an in-line gun color cathode ray tube, the proximal presence of the positive neck charge influence gradually causes the outer beams to move slightly outward from their normal paths. This change in beam positioning is noticeable as a gradual deterioration of the color imagery on the screen display wherein the edge definitions of the constituents of the display become gradually multi-colored and lose sharpness due to misplacement of focused beam impingement on the phosphor elements of the screen. This condition, which is referenced as static convergence drift, may develop to a degree necessitating readjustment of the exteriorly positioned beam correction devices or the operational circuitry related thereto. To minimize this condition, a known practice in the art has been to employ a substantially step-shaped metallic shield formed substantially of flat metallic material. This type of shielding was usually attached to the high voltage accelerating electrode in a manner to extend outward and slightly rearward therefrom to effect partial shielding of the interelectrode spacing between the focusing and accelerating electrodes. Such shielding, being positioned slightly outward from the electrodes, required a substantial amount of lateral spacing which is not available in many of the present tube constructions. In addition, this flat-type of drift shield, being formed substantially of sheet material, was prone to evidence an existence of sharp projections which were conducive for the possible generation of deleterious arcing within the gun structure.